Uniformity of Al2O3-ZrO2 Composites by Colloidal Filtration

Dispersion states of aqueous composite Al₂O₃/ZrO₂ colloidal suspensions were studied by measuring particle size distribution as a function of pH. Mutual dispersion was achieved at pH values of 2.0 to 3.5. Consolidated composites formed by colloidal filtration reflected the uniformity of the colloidal state. The mean flexural strength (896 MPa) of the sintered compacts was 1.6 times that of bodies consolidated by isostatic pressing .     

Neutron Diffraction Measurements of the Residual Stresses in Al2O3-ZrO2 (CeO2) Ceramic Composites

High-resolution neutron powder diffraction was used to study the residual stresses in Al₂O₃-ZrO₂ (12 mol% CeO₂) ceramic composites containing 10, 20, and 40 vol% ZrO₂ (CeO₂). The diffraction data were analyzed using the Rietveld structure refinement technique. The analysis shows that for all samples, the CeO₂-stabilized tetragonal ZrO₂ particles are in tension and the Al₂O₃ matrix is in compression. For both the ZrO₂ particles and the Al₂O₃ matrix, the average lattice strains are anisotropic and increase approximately linearly with a decrease in the corresponding phase content. It is shown that these features can be qualitatively understood by taking into consideration the thermal expansion mismatch between the ZrO₂ and Al₂O₃ grains. Also, for all composite samples, the diffraction peaks are broader than the instrumental resolution, indicating that the strains in these samples are inhomogeneous. From an analysis of the refined peak shape parameters, the average root-meansquare strain, which describes the distribution of the inhomogeneous strain field, was determined. Finally, the average residual stresses were evaluated from the experimentally determined average lattice strains and compared with recent results of X-ray measurements on similar composites.     

Micromechanical Stresses in SiC-Reinforced Al2O3 Composites

Applying an Eshelby approach, the internal micromechanical stresses within an SiC-inclusion-reinforced (platelet to whisker geometries) polycrystalline alumina matrix composite were calculated. The results are compared to the experimental residual stress measurements of a SiC-whisker-reinforced Al₂O₃ by Predecki, Abuhasan, and Barrett and found to be in excellent agreement. The calculations are then extended to SiC-reinforced composites with polycrystalline mullite, silicon nitride, and cordierite matrices. It is concluded that the internal stresses are significantly influenced by the inclusion geometry as well as the thermoelastic differences between the inclusion and the matrix and also the volume fraction.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Computation of Residual Stresses in Quenched Al2O3

A method of computing the residual stress profile in quenched A1₂O₃ rods was developed. For these calculations, certain material parameters must be determined. Thus, strain rate was measured as a function of stress for 96% A1₂O₃ at 1300° to 1500°C, and the heat-transfer rates of cylindrical samples quenched in several media were determined. Using calculated temperature distributions and measured strain rates, plastic strains were computed for the entire quenching period, and these strains were converted to a profile of residual room-temperature stresses. The substantial increases in flexural strength observed in Al₂O₃ after it is quenched (thermally conditioned) are considered to originate in the residual compressive surface layers.     

Critical Microstructures for Microcracking in Al2O3-ZrO2 Composites

The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al₂O₃/ZrO₂ composites. The degree of transformation was found to be dependent on the volume fraction of ZrO₂ and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr0₂, the ZrO₂ particles were found to pin the Al₂O₃ grain boundaries, thus limiting the Al₂O₃ grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO₂. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO₂) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO₂ at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO₂ grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO₂ size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO₂. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO₃ decoration technique, near the indentations.     

Comparison of KIc Values for Al2O3-ZrO2 Composites Obtained from Notched-Beam and Indentation Strength Techniques

Values of K_Icfor hot-pressed AL₂O₃-ZrO₂ composites were measured using notched-beam and indentation strength techniques. The results are compared with K factors at the mirror/mist boundary and at crack branching. It was found that the indentation strength technique provides a more consistent estimation of K_Ic, than the notched-beam technique.     

Fracture Strength–Fracture Resistance Response and Damage Resistance of Sintered Al2O3–ZrO2 (12 mol% CeO2) Composites

The fracture strengths of sintered Al₂O₃ containing 20 and 40 vol% ZrO₂(12 mol% CeO₂)—zirconia-toughened alumina (ZTA)—composites along with the fracture resistance can be increased (e.g., to ∼900 MPa and >12 Mpa·m^1/2, respectively), by increasing the mean grain size of the t -ZrO₂ (and the Al₂O₃) from ∼0.5 μm to ∼3 μm. At these lower t -ZrO₂ contents, the fracture strength-fracture resistance curves show a continuous rise as opposed to the strength maxima observed in polycrystalline t -ZrO₂(12 mol% CeO₂), CeTZP, and ZrO₂(12 mol% CeO₂) ceramics containing ≤20 vol% Al₂O₃. The toughened composites also exhibit excellent damage resistance with fracture strengths of 500 MPa retained with surfaces containing ∼150- N Vickers indentations which produce cracks of ∼160-μm radius. Greater damage resistance correlates with an increase in the apparent R -curve response of these composites.     

Dynamic Compaction of Al2O3-ZrO2 Compositions

Shock compaction of Al₂O₃-ZrO₃ binary and ternary powder compositions resulted in dense, one-piece samples without visible cracks for pressures ≤12.6 GPa. Dynamic pressures were achieved by using a 6.5-m-long two-stage gas gun. It is believed that plastic deformation by dislocation slip of α-Al₂O₃ partially accommodates the tensile stresses created during the release of shock pressures. A fine and narrow particle size distribution is necessary to achieve high bulk densities, but the bulk structural integrity was not strongly related to the distribution. A high-pressure phase of ZrO₂, which was formed from the monoclinic polymorph, was found at and above shock pressure of 6.3 GPa. No evidence of the orthorhombic cotunnite structure was observed. Compaction of glassy and submicrocrystalline rapidly solidified starting materials showed good structural integrity, although the bulk density was relatively low. It is not clear what the densificationhonding mechanism is in these materials, although it appears not to be plastic deformation. Microstructural analysis showed that fine and uniform microstructures are retained after compaction at appropriate dynamic pressures for all compositions, with some interparticle cohesion present.     

Bimodal Sintering of Al2O3/Al2O3 Platelet Ceramic Composites

The sintering behavior of an Al₂O₃ compact containing uniformly dispersed Al₂O₃ platelets has been investigated. The results reveal a significant decrease in the sintering rate as well as the formation of voids and cracklike defects in the presence of nonsinterable platelets. The addition of a small amount (2 vol%) of tetragonal-ZrO₂ particles enhances the sintering rate, increases end-point density (∼99.5% of theoretical density) and prevents formation of sintering defects.     

Determination of Residual Surface Stresses Caused by Grinding in Polycrystalline Al2O3

Residual surface stresses introduced into polycrystalline Al₂O₃ during diamond grinding (320 grit) were examined by an X-ray direction technique commonly used for metals. Compressive stresses, estimated to be in the range 135 to 170 MPa and to extend 15 μm deep, were observed at the surface. It is believed that the compressive surface layer coincides with the plastic layer produced by the elastie/plastic interaction of the abrasive grains with the ceramic. The results are discussed with regard to the effect of the compressive layer on the extension of surface cracks.     

Dynamic Fracture Characterization of Al2O3 and SiCw/Al2O3

The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al₂O₃ ceramic and 29 vol% SiC_w/Al₂O₃ composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced.     

Indentation Damage and Residual Stress Field in Alumina-Y2O3-Stabilized Zirconia Composites

Fracture features, residual stresses, and zirconia transformation are studied in indentation strength specimens of alumina-Y₂O₃-stabilized zirconia (3% mol of Y₂O₃, 3YTZP) ceramics in order to analyze the extension of the indentation damage in the bulk of the specimens. Two compositions, 5 vol% 3YTZP (A5) and 40 vol% 3YTZP (A40), have been prepared by stacking tape-casted tapes and sintering. After indentation with loads ranging from 50 to 300 N, samples were fractured in four-point bending and the fracture surfaces were characterized by scanning electron microscopy. Raman and piezospectroscopic techniques were used to determine the monoclinic zirconia fraction and the residual stresses through the fracture surfaces. In the A5 composition, the indentation damage morphology was clearly half-penny, whereas the A40 composition presented Palmqvist crack formation. Zirconia transformation was only observed in the plastically deformed zones underneath the imprints whereas there were significant residual compressive stresses outside the plastic zones due to the indentation damage. The intensity of this residual compressive field was dependent on the level of zirconia transformation due to indentation damage because zirconia transformation induced tensile stress fields superimposed on the compressive stresses.     

Microstructure Development in Al2O3-Platelet-Reinforced Ce-ZrO2/Al2O3 Composites

Composites containing Ce-ZrO₂, Al₂O₃, and aligned Al₂O₃ platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing.     

Indentation of Silicate-Glass Films on Al2O3 Substrates

The deformation of thin layers of glass on crystalline materials has been examined using newly developed experimental methods for nanomechanical testing. Continuous films of anorthite (CaAl₂Si₂O₈) were deposited onto Al₂O₃ surfaces by pulsed-laser deposition. Mechanical properties such as Young's modulus and hardness were probed with a high-resolution depth-sensing indentation instrument. Nanomechanical testing, combined with AFM in situ imaging of the deformed regions, allowed force-displacement measurements and imaging of the same regions of the specimen before and immediately after indentation. This new technique eliminates any uncertainty in locating the indentation after unloading. Emphasis has been placed on examining how the Al₂O₃ substrate crystallographic orientation will affect mechanical composite response of silicate-glass film/Al₂O₃ system.     

High-Temperature Strength and Fracture Toughness of Y2O3-Partially-Stabilized ZrO2/Al2O3 Composites

The temperature dependence of bending strength, fracture toughness, and Young's modulus of composite materials fabricated in the ZrO₂ (Y₂O₃)-Al₂O₃ system were examined. The addition of A1203 enhanced the high-temperature strength. Isostatically hot-pressed, 60 wt% ZrO₂ (2 mol% Y₂O₃)/40 wt% Al₂O₃ exhibited an extremely high strength, 1000 MPa, at 1000°C.     

Interfaces in Oriented Al2O3-ZrO2 (Y2O3) Eutectics

Oriented samples of Al₂O₃-ZrO₂ (Y₂O₃) eutectics consisting of an alumina matrix with zirconia dispersoids were grown by directional solidification. Preferred growth directions and epitaxial relations were determined from X-ray and electron diffraction analyses. Imaging of interfaces was performed by high-resolution transmission electron microscopy on oriented platelets. Semicoherent interfaces were observed with faceting along crystallographic planes of both phases.     

Influence of MgO Additiions on the Microstructure and Mechnical Properties of Al2O3-ZrO2 Composites

The effect of MgO and ZrO₂ dopants, added separeately or simultaneously, on the grain size, denisity, and toughness of Al₂O₃ was studied. Small ZrO₂ addotoopms (<100 ppm) had little effect, whereas larger amounts decreased the sintered density. Additions of Mgo Up to the solubility limit (∼ 300 ppm) increased both density and grain size; further additions had little effect on the density but strongly reduced grain size.     

Strength and Fracture Toughness of Isostatically Hot-Pressed Composites of Al2O3 and Y2O3-Partially-Stabilized ZrO2

Composites of Al₂O₃ and Y₂O₃ partially-stabilized ZrO₂ were isostatically hot-pressed using submicrometer powders as the starting material. The addition of Al₂O₃ resulted in a large increase in bending strength. The average bending strength for a composite containing 20 wt% Al₂O₃ was 2400 MPa, and its fracture toughness was 17 MN·w^−3/2